1. Field of the Invention
The present relates in general to upconversion luminescence (“UCL”) materials and methods of making and using same and more particularly, but not meant to be limiting, to Mn2+ doped semiconductor nanoparticles for use as UCL materials. The present invention also relates in general to upconversion luminescence including two-photon absorption upconversion, and potential applications using UCL materials, including light emitting diodes, upconversion lasers, infrared detectors, chemical sensors, temperature sensors, pressure sensors, ultraviolet and radiation detectors and biological labels, all of which incorporate a UCL material.
2. Brief Description of the Related Art
The ever increasing demands of electronics and the interface between electronic and biological systems pushes frontier of electrical sciences and, in particular, the size and energy consumption of electronic elements. As the particle size of a material gets smaller and smaller, novel specific phenomena can be observed such as the shift of emission to shorter wavelength with decreasing size. Upconversion luminescence (UCL) is a type of fluorescence wherein the excitation wavelength entering a material capable of exhibiting UCL is longer than the emission wavelength of such a UCL material. Due to potential applications in lasers, laser cooling, optical communications, storage, displays, imaging techniques, optical sensing and biological probing, upconversion luminescence has been extensively investigated. See for example the references cited hereinafter, the contents of which are expressly incorporated herein by reference in their entirety.
UCL is extensively documented, in particular, in rare earth compounds in which the presence of more than one metastable excited f-f state results from the efficient shielding of the 4f electrons, including the upconversion luminescence of Er3+ in BaTiO3 nanoparticles. Recently, upconversion has been reported in some transition metal compounds as well—see e.g. H. U. Gudel et al., New photon upconversion processes in Yb3+ doped CsMnCl3  and RbMnCl3, Chemical Physics Letters, (2000), 320, 639, hereinafter, the contents of which are expressly incorporated herein in their entirety. The upconversion luminescence of Mn2+ has been reported in Yb3+ doped CsMnCl3 and RbMnCl3 compounds at temperatures below 100 K.
Searching for an upconversion material, we need to consider both the stability and the upconversion efficiency. According to the energy gap law (J. M. F. van Dijk and M. F. H. Schuurmans, On the nonradiative and radiative decay rates and a modified exponential energy gap law for 4f-4f transitions in rare-earth ions, J. Chem. Phys. 1983, 78: 5317-5323 and L. A. Riseberg and H. W. Moos, Multiphonon orbit-lattice relaxation of excited states of rare-earth ions in crystals, Phys. Rev. 1968, 174: 429), lower highest lattice phonon energies of an upconversion host can reduce the rate, at which a given energy gap is crossed non-radiatively by multiphonon relaxation. That is, the lower the highest lattice phonon energy, the higher the upconversion efficiency. Most investigations of upconversion focus on oxides and halides. Oxides are air-stable but upconversion luminescence in oxides is not efficient because the phonon energies in oxides are high (most are higher than 500 cm−1) (P. Egger and J. Hulliger, Optical materials for short wavelength generation, Coordination Chemistry Review, 1999, 183: 101-115) and, thus, non-radiation rate is large, resulting in weak upconversion luminescence. In halides, the phonon energies is low (less than 350 cm−1) (P. Egger and J. Hulliger, Optical materials for short wavelength generation, Coordination Chemistry Review, 1999, 183: 101-115), non-radiation rate is low, and, of course, upconversion luminescence efficiency is high. However, halides are not stable in air because they are more or less sensitive to moisture. Group II-VI semiconductors like ZnS are very stable and their phonon energies are somewhat higher than that of halides but much lower than that of oxides (For ZnS:Mn2+, the phonon energy is 350 cm−1) (A. Anastassiadou et al., The luminescence spectrum of Zn1-xMnxS under hydrostatic pressure, Solid State Communications, 1988, 67:633-636). So, Group II-VI doped semiconductors are promising materials for upconversion due to their possibility of high upconversion efficiency and good stability. For example, the ZnS:Mn2+ nanoparticle samples used in the testing and experiments of the presently claimed and disclosed invention were made in approximately 2000, but as of the filing date of the present application still retain high luminescence and a stable structure.
Mn2+ doped semiconductors represent a class of phosphors that have already been utilized for many applications; however, no one has yet been able to report and/or demonstrate upconversion luminescence of Mn2+-doped semiconductors.
The presently claimed and disclosed invention(s) is predicated upon the observance of upconversion luminescence of Mn2+ in ZnS:Mn2+ semiconductor nanoparticles at room temperature. This upconversion luminescence is also shown to be principally due to two-photon excitation. These Mn2+-doped nanoparticles exhibiting UCL have unique and novel application as light emitting diodes, laser, optical communications, optical storage, infrared detection and imaging, chemical sensors, temperature and pressure sensing, radiation detection and biological probing. Additionally the presently disclosed and claimed invention provides for a novel methodology of distinguishing Mn2+ ions at the lattice sites and the near-surface sites in nanoparticles.
A common use of upconversion is to convert longer wavelength (infrared) to shorter wavelength (visible) and/or from a low energy to a higher energy state. Upconversion luminescence of Mn2+ is more efficient than that of rare earth ions because the d-d transition of Mn2+ can be modified via crystal field and in nanoparticles can become allowed or partly allowed, while the manipulation of the f-f transition of rare earth ions is more difficult and can only be slightly improved by crystal field or the host environments.
For infrared imaging, the use of nanoparticle upconversion is beneficial due to the low or absence of light scattering, because the light scattering intensity is proportional to the 6th power of the particle size (IμR6) (M. Kaszuba, The measurement of nanoparticles using photon correlation spectroscopy and avalanche photo diodes, Journal of Nanoparticle Research, (1999), 1, 405-409). Thus, compared to traditional micrometer-sized phosphors, light scattering in nanoparticles is nonexistent, which is ideal for imaging technology.
The surface-to-volume ratio of nanoparticles is very high. The attaching of chemicals or molecules to the nanoparticle surfaces changes the luminescence properties (intensity, emission energy and lifetime) of the nanoparticles greatly and rapidly. This provides for a new type of chemical sensors based on upconversion luminescence with high sensitivity.
The sizes of nanoparticles are comparable to the sizes of bio-molecules, and nanoparticles are soluble in water due to their small size and surface modification. Thus, highly luminescent nanoparticles are good labels for biological probing because they can combine with bio-molecules like antigens, anti-bodies, proteins or DNA or they can be inserted into biological systems such as a human or cell tissue. Some work has been done using undoped semiconductor nanoparticles. Until the present disclosure, however, no group has reported, discussed, or disclosed the use of doped nanoparticles and upconversion nanoparticles for biological systems. The application of undoped nanoparticles is based on photoluminescence in which the luminescence background and noise are very high due to the auto-fluorescence of the bio-molecules under ultraviolet excitation. Upconversion luminescence with an infrared excitation and a visible emission can avoid this shortcoming and thus improve the resolution and sensitivity of the upconversion luminescence assembly or molecule used, for example, as a biological sensor, probe or label.
At least one group (D. A. Zarling, M. J. Rossi, N. A. Peppers, J. Kane, G. W. Faris, M. J. Dyer, S. Y. Ng, and L. V. Schneider, Up-converting reporters for biological and other assays using laser excitation techniques, U.S. Pat. No. 5,891,656, Apr. 6, 1999) has reported using upconversion of the traditional micrometer sized phosphors for biological probing, but these molecules are not soluble in water and do not easily combine with biomolecules. The size, solubility and the strong upconversion luminescence of the doped nanoparticles of the presently claimed and disclosed invention, however, allow for the production and use of high quality biological probing materials and methodologies. Upconversion luminescence of doped nanoparticles has advantages over both the photoluminescence of undoped nanoparticles and the upconversion of traditional phosphors. In addition, nanoparticles can reduce light scattering intensity as discussed above, improving resolution greatly.
Temperature and pressure sensors can also be manufactured using the UCL materials of the presently claimed and disclosed invention. The UCL materials are especially well suited for temperature sensors due to the sensitivity of the upconversion luminescence intensity to temperature as shown in detail hereinafter. For example, there exists a linear or a near linear relationship between the upconversion intensity and temperature for high quality nanoparticles like the ZnS:Mn2+ nanoclusters encapsulated in zeolite. In addition, upconversion spectra, lifetime and temperature dependence measurements demonstrate that upconversion is a good method to reveal the luminescence characteristics of the Mn2+ ions at the near-surface sites of semiconductor nanoparticles.